Commenting on the younger pupil's arrest, Makhubele said: "Police formed a team of detectives who followed up the information from the scene, which led them to another school in Mondeor, where one 13-year-old suspect is a pupil."

The 15-year-old boys were arrested in Soweto and Naturena.

Police had earlier called on the pair to hand themselves over but Makhubele said they had instead been arrested at their homes in the early hours of Thursday morning.

It was not clear whether any of the suspects had known Mathebula prior to the attack.

The trio is expected to appear in the Johannesburg Magistrate's Court on Friday. Because they are minors, the proceedings will most likely be held in camera.

A white cross with flowers has been erected at the scene where Kulani Mathebula drew his last breath. He was allegedly attacked and stabbed several times on his chest by 3 unidentified persons. Police have arrested a 13 year old and are searching for 2 more. #MondeorStabbingpic.twitter.com/dmqDBezDPh